Catalytic cracking of hydrocarbon oils over crystalline silicate catalysts is well known for the efficient conversion of hydrocarbons into useful and very valuable products including high octane gasoline, cycle oil and olefins. The cracking processes are basically conducted in a fluid bed catalytic cracking unit (FCC) or a thermofor catalytic cracking reactor (TCC). In general, TCC is a continuous process using a moving bed of solid catalyst in bead form which flow through the reactor where cracking takes place and then is taken to a kiln where coke deposits from the cracking reactions are burned off. In the FCC unit a solid catalyst in the form of a fine powder is cycled between a reactor and a regenerator. Vaporous hydrocarbons which are bubbled through the powdered catalyst make the catalyst flow like a liquid.
The spectrum of products produced by catalytic cracking is broad both in terms of molecular weight and chemical class. For example, a heavy gas oil will yield C.sub.5 +-400.degree. F. gasoline as a major product, light fuel oil, heavy fuel oil, C.sub.4 's, C.sub.3 -gas and coke. Only limited quantities of the light hydrocarbons can be used in gasoline because of their high vapor pressure. However, the light olefins produced by cracking are important products because they can be upgraded by various light olefin upgrading processes.
The light olefins are useful as a feed for methyl-tert-butyl-ether (MTBE) and tert-amyl-methyl ether (TAME) synthesis and alkylation processes which lead to an overall increase in the refinery gasoline pool.
Additionally, new laws which mandate a higher content of oxygenated compounds in gasoline require refiners to maximize refinery output of light olefins. The light olefins, isobutylenes and isoamylenes, used to produce (MTBE) and (TAME), are the oxygenated gasoline blending components of choice for reformulated gasolines. A proper formulation of catalyst composition and cracking operation conditions can significantly effect the light olefins output. Furthermore, the use of MTBE and TAME as gasoline additives imparts excellent octane gain to both premium and regular gasoline blends.
Additionally, the low molecular weight products can be used to produce high octane blending components to improve the refinery gasoline yield. The low molecular weight products of catalytic cracking can be used to make the highly branched paraffins which have good octane properties by a building-up process known as paraffin alkylation, or, simply, alkylation. The motor octane rating of the products from alkylating the isobutane with the propylene, butylene, and amylene light products of cracking reactions are very good, i.e., 89, 93 and 90, respectively.
Moreover, government regulations which mandate stringent gasoline specifications increase the importance of production of alkylate gasoline. In addition to the enhanced octane, alkylate can help reduce vehicle emissions as the components in the alkylate do not contribute to ozone formation. Also, alkylate has low vapor pressure which allows refiners to maintain government mandated volatility specifications. See L. F. Albright, "Alkylation will be Key Process in Reformulated Gasoline Era", Oil & Gas Journal, Nov. 12, 1990, pp. 79-92.
The types of catalysts chosen for use in cracking reactions can influence the efficiency of the process and can effect the product distribution.
Crystalline zeolites, both natural and synthetic, are particularly effective for catalytic cracking reactions. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials have come to be known as "molecular sieves" and are utilized in a variety of ways to take advantage of these properties. Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as rigid three-dimensional frameworks of SiO.sub.4 and Periodic Table Group IIIA element oxide, e.g., AlO.sub.4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIA element, e.g., aluminum, and silicon atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIA element, e.g., aluminum, is balanced by the inclusion in the crystal of a cation, e.g., an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIIA element, e.g., aluminum, to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing conventional ion exchange techniques. By means of cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation. The spaces between the tetrahedra are occupied by molecules of water prior to dehydration.
Prior art techniques have resulted in the formation of a great variety of synthetic zeolites. Many of these zeolites have come to be designated by letter or other convenient symbols, as illustrated by zeolite Z (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite ZK-5 (U.S. Pat. No. 3,247,195); zeolite ZK-4 (U.S. Pat. No. 3,314,752); zeolite ZSM-5 (U.S. Pat. No. 3,702,886); zeolite ZSM-11 (U.S. Pat. No. 3,709,979); zeolite ZSM-12 (U.S. Pat. No. 3,832,449); zeolite ZSM-20 (U.S. Pat. No. 3,972,983); zeolite ZSM-35 (U.S. Pat. No. 4,016,245); and zeolite ZSM-23 (U.S. Pat. No. 4,076,842), to name a mere few.
The SiO.sub.2 /Al.sub.2 O.sub.3 ratio of a given zeolite is often variable. For example, zeolite X can be synthesized with SiO ratios of from 2 to 3; zeolite Y, from 3 to about 6. In some zeolites, the upper limit of the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is unbounded. ZSM-5 is one such example wherein the SiO.sub.2 /Al.sub.2 O.sub.3 ratio is at least 5 and up to the limits of present analytical measurement techniques. U.S. Pat. No. 3,941,871 (Re. 29,948) discloses a porous crystalline silicate made from a reaction mixture containing no deliberately added alumina in the recipe and exhibiting the X-ray diffraction pattern characteristic of ZSM-5. U.S. Pat. Nos. 4,061,724, 4,073,865 and 4,104,294 describe crystalline silicates of varying alumina and metal content. Varying the silica-to-alumina ratio has been demonstrated to influence the range and distribution of products of catalytic cracking.
Zeolite Beta is a known zeolite which is described in U.S. Pat. Nos. 3,308,069 and RE 28,341 both to Wadlinger, and reference is made to these patents for a general description of zeolite Beta. The zeolite Beta of Wadlinger is described as having a silica-to-alumina ratio of 10 to 100 and possibly as high as 150.
A catalyst system incorporating zeolite Beta as an additive catalyst along with a faujasite catalyst is described in U.S. Pat. No. 4,740,292 as being effective for upgrading the total yield and octane number of the gasoline product. The higher silica zeolite Beta is described as performing well in the conversion processes (i.e. silica-to-alumina ratio above 200:1).
A highly silicious zeolite Beta made by direct synthesis is described in U.S. Pat. No. 4,923,690 to Valyocsik; however, to achieve high silica-to-alumina ratios the zeolite is only partially crystalline. Thus, as described in the patent, when the crystallinity of the zeolite increases, the silica-to-alumina ratio decreases. For purposes of catalytic cracking, it would be desirable to have a fully crystalline zeolite in which the silicate is free of amorphous materials.
The description of the zeolite Beta synthesized by Wadlinger is silent as to the crystallite size. Typically, however, the zeolite Beta produced by the Wadlinger method is a small crystal having a crystal size ranging from 0.01 to 0.05 microns. Large crystal zeolites have been found to possess distinct advantages in certain hydrocarbon conversion processes over the smaller crystal zeolites.
Larger crystal zeolites are known to provide longer diffusion path lengths which can be used to modify catalytic reactions. By way of illustration only, in the medium pore zeolite ZSM-5, manipulating crystal size in order to change the selectivity of the catalyst has been described. A unique shape selective characteristic of ZSM-5 is the para-selectivity in toluene disproportionation and aromatics alkylation reactions. Increasing the size of the crystal, thereby lengthening the diffusion path, is just one way of achieving a high para-selectivity. The product selectivity occurs because an increase in the diffusion constraints is imposed on the bulkier, slower diffusing o- and m- isomers which reduces the production of these isomers and increases the yield of the para-isomer. N.Y. Chen et al, Shape Selective Catalysis in Industrial Applications, p.p. 51 (Marcel Dekker, Inc New York 1989) and N.Y. Chen et al, Industrial Application of Shape Selective Catalysts, p.p. 196 (Catal. Rev. Sci. Eng. 28 (2&3) 1986). Obtaining high selectivities in zeolite ZSM-5 by increasing the crystal size is described in U.S. Pat. No. 4,517,402 which is incorporated herein by reference. In U.S. Pat. No. 4,828,679 it is revealed that large crystal ZSM-5 type zeolites have improved octane gain and total motor fuel yield as well as improved steam stability. U.S. Pat. No. 4,650,656 describes a large crystal ZSM-5 which is synthesized by controlling the reaction conditions such as the rate of addition of the organics, the temperature, pH and the degree of agitation of the crystallization media. The application of an external gravitational force during the synthesis of silicalite has been described as a means for producing a large crystal zeolite in D. T. Hayhurst et al, "Effect of Gravity on Silicalite Crystallization" in Zeolite Synthesis p.p. 233 (M. L. Occelli Ed. American Chemical Society 1956). In J. F. Charnell, "Gel Growth of Large Crystals of Sodium A and Sodium X Zeolites", Jour. Crystal Growth 8, pp. 291-294, (North Holland Publishing Co., 1971), a method of synthesizing large crystal zeolite A and zeolite X is described in which, as the only organic reactant, triethanolamine is incorporated into the reaction mixture. A review of these publications reveals that a significant amount of attention has been directed to synthesizing large crystal zeolites yet none of the publications point to a consistent method for producing the large crystals. The crystal size of zeolite Beta was generally related to the silica-to-alumina mole ratio, the highly silicious zeolite Beta corresponding to a larger crystal size and the lower silica-to-alumina mole ratio corresponding to a smaller crystal size. Techniques for synthesizing a large crystal zeolite Beta covering a broad range of silica-to-alumina ratios, including the high as well as the low silica-to-alumina ratios, would be desirable and the advantages would be appreciated in hydrocarbon conversion processes.